A method for determining the association constant of tryptophan-containing peptides with the ions of copper, nickel, and zinc has been developed. The technique measures the quenching of tryptophan fluorescence upon binding. The quenching is complete when Cu++ is bound, and partial when Ni++ binds. Zn++ does not quench the fluorescence of these peptides, but its association constants can be obtained by competition with either copper or nickel. As part of these binding studies, it was necessary to measure the ionization constants for the amino group. This was done by following fluorescence as a function of pH, coupled with curve fitting. A series of fluorescence standards being developed by the National Bureau of Standards was evaluated. These new standards consist of sintered mixtures of inorganic phosphors and polytetrafluoroethylene resin. The standards are solid samples which emit over the wavelength region 400 to 700 nm. Because the corrected spectra of these standards are known, it is in theory possible to calibrate detector systems with the standards. Use of the standards for this purpose, their stability, and their usefulness as standards to eliminate instrumental nonlinearities are being evaluated.